I don't have the luxury of garnering access to a mechanical track cleaning locomotive. But I will explain what I do.
Plastic wheels leave a mysterious residue behind on the brass rail. Mysterious is why they use a polymer which would do that to make the wheels, when inexpensive plastics that don't smear are available. Wiping and rubbing the track with alcohol on a sock was not getting this residue off the rail, so I took a really fine, soft sanding bit on my dremel tool and simply buffed the residue off of the rail. It worked great, was kind of fun to do, but it does take a minute amount of brass in the process and I did not like the prospect of doing this again.
Then I discovered that using mineral spirits on a rag or sock, will easily remove the plastic residue because the mineral spirits eats plastic! But one thing to remember is that this could affect the precious plastic ties under the track, if enough is used for a long period of time. Mineral spirits are fairly cheap to get at your hardware store, and turpentine probably has a similar effect. These paint thinners go after petroleum products like paint or plastic.